This disclosure is directed at skeletal bone fixation systems, and more particularly to a fixation assembly for vertebrae of a spinal column.
Spinal fixation systems are used to secure sections of the spinal column, such as vertebral bodies, into a fixed position to correct spinal injuries and defects. Internal fixation is used most frequently in the spine in conjunction with vertebral fusion, and also for the manipulation of the spine to correct spinal deformities. A typical spinal fixation assembly includes a fixation device, such as a screw or hook, that can be attached to a portion of a first vertebral body. The screw can be coupled to a stabilization member, such as an elongate rod, that can be linked to one or more additional vertebral bodies using additional screws.
Pursuant to a general process, two or more bone screws and/or hooks are secured to a vertebral body that is to be stabilized. After the screws are secured to the vertebral bodies, the screws are coupled to a spinal stabilization rod that restricts movement of the stabilized vertebra. It is important that the screws have a secure coupling with the spinal stabilization rod in order to prevent movement of the rod relative to the screw after placement.
In several available pedicle screw systems, a tulip-like coupling element with opposing upright arms or walls is used to secure the pedicle screw to the rod. The coupling element and pedicle screw are configured to be coupled to an elongate stabilizer, such as a rod, that is positioned above the head of the pedicle screw. A compression member, such as a compression nut, is configured to mate with the coupling element and provides a compressive force to the rod. The rod is then forced against the head of the pedicle screw, and that force is translated to the coupling element. Accordingly, the forces generated by the compression nut clamp the rod and pedicle screw head together within the coupling element.
One of the problems with this type of arrangement has been that the shape of the rod and the shape of the pedicle screw head are typically such that the amount of surface area contact between the two is limited. Rods are usually cylindrical and pedicle screw heads are usually either flat or hemispherical. The resulting contact area is relatively small, increasing the potential for slippage and failure in the pedicle screw system.
Another problem is that the upright legs or walls of the coupling element can experience splaying after implantation. Significant splaying of the arms generally results in failure of the coupling element, since the compression member or nut can no longer be retained in the coupling element to clamp the rod against the pedicle screw head. As a result, the rod is free to move relative to the coupling element, causing a failure that reduces or eliminates the effectiveness of the pedicle screw system.
Yet another problem is that the forces exerted on the coupling element can cause minute movement or rotation in the compression nut. As a result, the clamping force on the rod is reduced, potentially causing a failure in the pedicle screw system that can reduce or eliminate the effectiveness of the system.
Pedicle screw implantation procedures are costly, risky and result in painful and lengthy recovery for the patient. Thus, it is important that multiple surgeries to resolve failures in the implants be avoided. Furthermore, it can be a tedious process to position the screws on the vertebral bodies and to interconnect them with the stabilizing rod. Thus, it is desirable that the screws be easily attached to the rods and that, once attached, the coupling between the screw and rod be secure and not prone to failure. In view of the foregoing, there is a need for improved pedicle screw systems.